LNF:Fuelling and Impurity Control Studies in the stellarators TJ-II and W7-X using Cryogenic Pellets and Tracer-Encapsulated Solid Pellets (TESPEL)
LNF - Nationally funded project
Title: Fuelling and Impurity Control Studies in the stellarators TJ-II and W7-X using Cryogenic Pellets and Tracer-Encapsulated Solid Pellets (TESPEL)e
Reference: PID2020-116599RB-I00
Funding Umbrella: Plan Estatal de Investigación Científica y Técnica y de Innovación 2021-2023
Funding Programme: Proyecto de investigación subvencionado por el Ministerio de Ciencia e Innovación
Subprogramme: Proyectos de I+D+i Retos Investigación
Programme and date: Proyectos I+D+i 2020
Programme type/ Modalidad:
Area/subarea: Physical Sciences / Physics and its applications
Principal Investigator(s): Kieran Joseph McCarthy María Isabel García Cortés
Project type: Investigación Orientada Tipo B
Start-end dates: 01/09/2021 - 31/08/2024
Financing granted (direct costs): 130000 €
Description of the project
The goal of this project, which falls within the realm of magnetic confinement nuclear fusion, is to continue research initiated in projects ENE2013-48679-R and FIS2017-89326-R on fuelling and impurity control of plasmas in the stellarators TJ-II (Ciemat, Madrid) and W7-X (Greifswald, Germany). Further research to resolve these issues is critical to demonstrate steady-state operation of helical-type fusion reactors. Thus it will contribute to the development and scientific exploitation of stellarators, a priority highlighted in the document "Fusion Electricity: a roadmap to the realization of fusion energy" (EFDA 2012).
The first aim is to investigate aspects of plasma fuelling that are still not fully understood and the effects of fuel pellets on plasma magnetic activity, plasma turbulence and plasma performance. For this, the medium-sized heliac TJ-II will be used. It is equipped with a cryogenic pellet injector (PI) for producing solid hydrogen pellets. It is intended to evaluate fuelling efficiency across a broad range of magnetic configurations and to identify and explore new pellet phenomena. While TJ-II is equipped with a large number of modern diagnostics, it is proposed to develop a new filterscope-based system to measure pellet cloud density and temperature to extend knowledge of pellet physics.
The second aim is to continue to support impurity transport and accumulation studies in TJ-II and W7-X. In a trilateral collaboration with National Institute for Fusion Science (Japan) and IPP-Max-Planck (Greifswald, Germany), Tracer-Encapsulated Solid Pellet (TESPEL) injections systems have been installed on both TJ-II and W7-X. TESPELs are polystyrene spheres (diameter <1 mm) loaded with impurity tracers (elements other than fuel). This allows delivering a precise quantify of tracer to a preselected location in the plasma core. An important aspect of the collaboration has been the establishment of a laboratory to fabricate TESPELs at Ciemat for both devices (FIS2017- 89326-R). Key parts of this proposal are to continue TESPEL fabrication at this laboratory, thereby allowing Ciemat to maintain this fruitful collaboration, and to upgrade a vacuum ultraviolet spectrometer on TJ-II to provide important spectral line data for impurity identification in W7-X.
The PI and TESPEL systems on TJ-II share a common injection guide lines. This unique set-up allows direct comparative studies of ablation, deposition and plasma response to be made thereby facilitating the understanding of common physics. Given that fuelling and impurity control are critical issues for stellarator steady-state operation, the project will allow us to continue to contribute to, and participate in, research programmes on W7-X, the stellarator of reference. Finally, team members have significant experience in the formation of young researchers at Master and PhD levels and in disseminating research to second level students and to the general public.
Main Results
1. A pellet-induced enhanced confinement regime (or PiEC) has been identified in NBI-heated discharges made in TJ-II after the injection of a single cryogenic fuel pellet into the plasma core [1]. In addition to the expected increase in core electron density, the plasma diamagnetic energy content, as determined using a diamagnetic loop, is seen to rise by up to 40%, with respect to reference discharges without pellet injection. Furthermore, the energy confinement time is significantly enhanced when compared to predictions obtained using the 2004 International Stellarator Scaling law, ISS04. Indeed, the operational regimes of other stellarator devices, such as LHD and W7-X, can be similarly extended to performance well beyond those obtainable with gas puffing alone.
2. Additional studies, performed with multiple pellet injections, have extended the TJ-II operational regime well beyond limits previously achieved in this device using NBI heating and gas puff. In order to achieve best results, the plasma target electron density should to be in the range 1x10^1919 to 2.5x10^19 m^-3 and time separations between pellets should be close to energy confinement times, around 10 ms.
References
[1] Enhanced confinement induced by pellet injection in the stellarator TJ-II I. García-Cortes, K. J. McCarthy, T. Estrada, V. Tribaldos, B. van Milligen, E. Ascasíbar, R. Carrasco, A. A. Chmyga, R. García, J. Hernández-Sánchez, C. Hidalgo, S. Kozachek, F. Medina, D. Medina-Roque, M. A. Ochando, J. L. de Pablos, N. Panadero, I. Pastor and TJ-II Team, Phys. Plasmas 30 (2023) 072506. https://doi.org/10.1063/5.0151395
[2] Multi-pellet injection into the NBI-heated phase of TJ-II plasmas, K. J. McCarthy, I. García-Cortés, A. Alonso, A. Arias-Camisón, E. Ascasíbar, A. Baciero, A. Cappa, R. Carrasco, O. O. Chmyga, T. Estrada, R. García, J. Hernández-Sánchez, F. J. Herranz, O. S. Kozachok, B. López Miranda, F. Medina, D. Medina-Roque, B. van Milligen, M. Navarro, M. A. Ochando, J. L. de Pablos, N. Panadero, I. Pastor, J. de la Riva, M. C. Rodríguez, D. Tafalla, V. Tribaldos and TJ-II Team, Nucl. Fusion 64 (2024) 066019, https://doi.org/10.1088/1741-4326/ad4047.